Clean up some code quality issues

This removes some qualifiers that aren't necessary (e.g. public/abstract on interfaces, private on enum constructors, final on private methods, static on nested interfaces/enums), uses Java 8 lambdas and features where that's an improvement

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=177182945

java/google/registry/flows/domain/DomainTransferRequestFlow.java

@@ -197,9 +197,7 @@ public final class DomainTransferRequestFlow implements TransactionalFlow {
             existingDomain,
             trid,
             gainingClientId,
-            (feesAndCredits.isPresent())
-                ? Optional.of(feesAndCredits.get().getTotalCost())
-                : Optional.empty(),
+            feesAndCredits.map(FeesAndCredits::getTotalCost),
             now);
     // Create the transfer data that represents the pending transfer.
     TransferData pendingTransferData =